A human eye is subject to a variety of optical aberrations. Accurate measurement of optical aberrations is essential for precise correction by customized photo-refractive surgery, by use of customized contact lenses, or by use of customized intra-ocular lenses. Wavefront measurement is a commonly used method to determine optical aberrations of an eye, monitoring penetrating keratoplasty (PKP) healing, or providing quantitative error data.
Shack-Hartmann (SH) wavefront systems are known in the art for measuring ocular aberrations. Starting at the retina, an ideal wavefront is generated which passes through the optical path of the eye. As the wavefront exits the eye, it contains a map of the eye's aberrations for analysis by a lenslet array of a SH sensor. A lenslet array dissects the incoming light into a large number of sub-apertures, and then measures the wavefront slope across each sub-aperture. The sensor information is used to analyze the optical properties of the system which created the wavefront of the eye.
SH wavefront sensors suffer from problems with intensity modulation that introduce scintillation effects and non uniformity of retinal reflection. For example, spot shapes on the Hartmannogram are very irregular, which makes the error of estimation of the spots' centers rather large. To overcome this problem, existing systems collect multiple images over time and average the multiple images or collect an image over a long period of time to smooth out the scintillation effects. However, temporal and spatial errors are introduced into the data with these solutions.
Occluded or damaged corneas produce scatter and/or scintillation beyond those found in normal eyes. Scatter and scintillation produce amplitude fluctuation in SH lenslet images rendering a wavefront that is known to be inaccurate. Therefore eyes with larger scatter and/or scintillation are excluded from aberration measurements using existing SH aberrometers. The accuracy and limitations for the SH sensor are discussed by Pedro M. Prieto et al. “Analysis of the performance of the Hartmann-Shack sensor in the human eye” J. Opt. Soc. Am. Vol. 17 No. 8, August 2000, pgs. 1396-1398.
Existing SH wavefront sensors have a narrow dynamic range and suffer from a lack of high sensitivity. Existing SH wavefront systems only work with relatively clear lenses and corneas. Currently, no wavefront system exists that works well to produce objective data for eyes with scintillation and scatter, over a wide dynamic range, or varying amounts of accommodation.